The present invention relates to a spiral antenna.
Four-arm spiral antennas are described in the book Four-Arm Spiral Antennas by R. G. Corzine, J. A. Moskos, Artech House, 1990.
The spiral antenna according to the present invention has the advantage over the related art that the spiral arms are connected on their respective inner spiral arm ends to a coplanar conductor for supplying and/or receiving signals. By using the coplanar conductor, it is possible to eliminate power supply networks for adjusting the phase angles at the incoming feed points of the spiral antenna or for making the electric field to be supplied symmetrical or asymmetrical, and thus to reduce costs.
Another advantage is that, due to the use of the coplanar conductor, the spiral antenna can be operated in a first mode to generate an omnidirectional transmission characteristic and also in a second mode to generate a directional transmission characteristic normal to the plane of the spiral. In this way, the spiral antenna can be used as a combination antenna for various wireless services.
It is especially advantageous that the coplanar conductor and the spiral antenna can be applied to different carrier materials. The transition from the coplanar conductor to the spiral antenna does not depend on any sudden change in the dielectric constant. Thus a carrier material having a low permittivity can be used for the spiral antenna, thus achieving a good transmission. At the same time, a carrier material having a high permittivity can be selected for the coplanar conductor, thus permitting a reduction in the length of the coplanar conductor while suppressing parasitic radiation from the coplanar conductor, so that the coplanar conductor can be made independent of the radiation field of the spiral antenna
Another advantage is that the coplanar conductor is designed with a taper at least in part. In this way, no additional network is necessary for adapting the impedance of the coplanar conductor to the input impedance of the spiral antenna.